Minor VCF parser improvements

Hello,

* `Biocaml_vcf` now supports reserved sub-fields for INFO and FORMAT.
* Also, I've implemented basic ALT parsing; currently everything is stringly typed, so, for example, deletion will be [represented](https://github.com/superbobry/biocaml/blob/v0.3-dev/src/tests/test_vcf.ml#L116) as `"<DEL>"` instead of an appropriate ADT constructor. I'm not yet sure how to approach the types for a zillion of variations listed in VCF standard, any ideas here?

By the way, is it okay to use `Hashtbl` for VCF parser state? the size of VCF header is usually small, so container choice doesn't affect efficiency, but using **immutable** `Map` looks like a cleaner solution. What do you think?